In image forming apparatuses to form images by an electro-photographic method, said apparatus forms a toner image on a sheet by using micro-powder toner, and heats and presses the toner image to permanently fix it onto the sheet.
This fixing process heats a sheet at high temperature so that any moisture included in the sheet evaporates, however since both surfaces of the sheet are not heated by the same amount of heat, different amounts of water evaporate from each surface, which is part of the reason for a curled sheet.
To overcome this problem, Unexamined Japanese Patent Application Publication No. 2000-247526 discloses a de-curling device which bends the curled sheet opposite to a curled direction. This device includes a curl-correction mechanism (which is so-called “de-curler”) incorporating paired rollers and a looped space, where the de-curler is switched to be used or not, based on the characteristics or type of the supplied sheet.
Further, a dried sheet, after passing through the fixing process is introduced to the ambient environment, from which it absorbs moisture so that the amount of moisture in said sheet gradually turns to be normal. However the amount of moisture returns at various rates, and the amount of moisture differs in various sections of the sheet, which results in a waved sheet.
To overcome this problem, Unexamined Japanese Patent Application Publication No. 2002-193524 discloses a device for applying water to a sheet, which includes a water supplying container to store water, and paired moisturizing rollers to receive water stored in the water supplying container, wherein when the sheet passes between the paired rollers, the sheet is moisturized.
However, said device moisturizes every supplied sheet, whereby moisturization is conducted on the sheets which are not waved or tend to be not waved. Accordingly, water consumption increases so that an operator frequently needs to replenish water in the container, which is very bothersome.
Further, if moisturization is conducted on the sheets which are not waved or tend to be not waved, said sheets contain so much water that they become thick, and its rigidity decrease, which tend to result in erratic sheet conveyance, or after said sheets are ejected to be stacked on a sheet ejection tray, they adhere to each other, which needs to be prevented.